A Comparative Mechanistic Study of the Reversible Binding of NO to a Water-Soluble Octa-Cationic FeIII Porphyrin Complex

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)FeIII(H2O)(HisH)] (1), [(AcMP-11)FeIII(OH)(HisH)] (2), and [(AcMP-11)FeIII(OH)(His-)] (3). From the pH dependence of the second-order rate constant for NO binding (kon), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (kon(1) = 3.8 × 106 M-1 s-1) to 2 (kon(2) = 4.0 × 105 M-1 s-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)FeIII(OH)(HisH)] (2a) and minor [(AcMP-11)FeIII(H2O)(His-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH2 bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH2 bond in the [(AcMP-11)FeIII(H2O)(His-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H2O and HisH ligands based on the experimental pKa values. It is shown that the "effective lability" of the axial ligand (OH-/H2O) in 2 may be comparable to the lability of the H2O ligand in 1.

High-Pressure Mechanistic Insight into Bioinorganic NO Chemistry

Pressure is one of the most important parameters controlling the kinetics of chemical reactions. The ability to combine high-pressure techniques with time-resolved spectroscopy has provided a powerful tool in the study of reaction mechanisms. This review is focused on the supporting role of high-pressure kinetic and spectroscopic methods in the exploration of nitric oxide bioinorganic chemistry. Nitric oxide and other reactive nitrogen species (RNS) are important biological mediators involved in both physiological and pathological processes. Understanding molecular mechanisms of their interactions with redox-active metal/non-metal centers in biological targets, such as cofactors, prosthetic groups, and proteins, is crucial for the improved therapy of various diseases. The present review is an attempt to demonstrate how the application of high-pressure kinetic and spectroscopic methods can add additional information, thus enabling the mechanistic interpretation of various NO bioinorganic reactions.

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

Nitrosyl- versus nitroxyl-cobalamin?

The Commentary is in answer to the comment of a reader that objected against the use of the term ‘nitroxylcobalamin’ in two recent reports in JBC from our group. We use this opportunity to explain to the reader where this terminology originated from.

Cationic porphyrins with large side arm substituents as resonance light scattering ratiometric probes for specific recognition of nucleic acid G-quadruplexes

Specific G-quadruplex-probing is crucial for both biological sciences and biosensing applications. Most reported probes are focused on fluorescent or colorimetric recognition of G-quadruplexes. Herein, for the first time, we reported a new specific G-quadruplex-probing technique-resonance light scattering (RLS)-based ratiometric recognition. To achieve the RLS probing of G-quadruplexes in the important physiological pH range of 7.4-6.0, four water soluble cationic porphyrin derivatives, including an unreported octa-cationic porphyrin, with large side arm substituents were synthesized and developed as RLS probes. These RLS probes were demonstrated to work well for ratiometric recognition of G-quadruplexes with high specificity against single- and double-stranded DNAs, including long double-stranded ones. The working mechanism was speculated to be based on the RLS signal changes caused by porphyrin protonation that was promoted by the end-stacking of porphyrins on G-quadruplexes. This work adds an important member in G-quadruplex probe family, thus providing a useful tool for studies on G-quadruplex-related events concerning G-quadruplex formation, destruction and changes in size, shape and aggregation. As a proof-of-concept example of applications, the RLS probes were demonstrated to work well for label-free and sequence-specific sensing of microRNA. This work also provides a simple and useful way for the preparation of cationic porphyrins with high charges.

Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

The redox reaction of superoxide (KO2) with highly charged iron porphyrins (Fe(P4+), Fe(P8+), and Fe(P8-)) has been investigated in the ionic liquids (IL) [EMIM][Tf2N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [EMIM][B(CN)4] (1-ethyl-3-methylimidazolium tetracyanoborate) by using time-resolved UV/vis stopped-flow, electrochemistry, cryospray mass spectrometry, EPR, and XPS measurements. Stable KO2 solutions in [EMIM][Tf2N] can be prepared up to a 15 mM concentration and are characterized by a signal in EPR spectrum at g = 2.0039 and by the 1215 cm(-1) stretching vibration in the resonance Raman spectrum. While the negatively charged iron porphyrin Fe(P8-) does not react with superoxide in IL, Fe(P4+) and Fe(P8+) do react in a two-step process (first a reduction of the Fe(III) to the Fe(II) form, followed by the binding of superoxide to Fe(II)). In the reaction with KO2, Fe(P4+) and Fe(P8+) show similar rate constants (e.g., in the case of Fe(P4+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 2.8 ± 0.1 M(-1) s(-1) for the second reaction step). Notably, these rate constants are four to five orders of magnitude lower in [EMIM][Tf2N] than in conventional solvents such as DMSO. The influence of the ionic liquid is also apparent during electrochemical experiments, where the redox potentials for the corresponding Fe(III)/Fe(II) couples are much more negative in [EMIM][Tf2N] than in DMSO. This modified redox and kinetic behavior of the positively charged iron porphyrins results from their interactions with the anions of the ionic liquid, while the nucleophilicity of the superoxide is reduced by its interactions with the cations of the ionic liquid. A negligible vapor pressure of [EMIM][B(CN)4] and a sufficient enrichment of Fe(P8+) in a close proximity to the surface enabled XPS measurements as a case study for monitoring direct changes in the electronic structure of the metal centers during redox processes in solution and at liquid/solid interfaces.

A pair of highly biotolerated diamagnetic and paramagnetic iron(ii) complexes displaying electroneutrality

A pair of structurally analogous macrocyclic iron(ii) complexes with a magnetic off-on relationship is reported that exhibit electroneutrality at neutral pH and high stability in physiological media.

Mechanistic studies on the reactions of cyanide with a water-soluble Fe(III) porphyrin and their effect on the binding of NO

The reaction of the water-soluble Fe(III)(TMPS) porphyrin with CN(-) in basic solution leads to the stepwise formation of Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2). The kinetics of the reaction of CN(-) with Fe(III)(TMPS)(CN)(H(2)O) was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of Fe(III)(TMPS)(CN)(2) is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of Fe(III)(TMPS)(CN)(OH) under the selected reaction conditions. Both Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2) bind nitric oxide (NO) to form the same nitrosyl complex, namely, Fe(II)(TMPS)(CN)(NO(+)). Kinetic studies indicate that nitrosylation of Fe(III)(TMPS)(CN)(2) follows a limiting dissociative mechanism that is supported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters ΔS(‡) and ΔV(‡) found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 ± 0.01) × 10(-2) s(-1), correlates with the rate constant for CN(-) release from the Fe(III)(TMPS)(CN)(2) complex, namely, (1.3 ± 0.2) × 10(-2) s(-1) at 20 °C, and supports the proposed nitrosylation mechanism.

Mechanistic insight into formation of oxo-iron(IV) porphyrin pi-cation radicals from enzyme mimics of cytochrome P450 in organic solvents

Two new models for cytochrome P450 in which the thiolate axial ligand is replaced by a RSO(3)(-) group, form oxo-iron(IV) porphyrin pi-cation radicals as sole oxidation products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings. Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates. This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron(IV) porphyrin pi-cation radical formation from the enzyme mimics of P450 for different oxidant concentration, temperature and pressure in selected organic solvents. Extraction of the appropriate rate constants and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equilibrium involving formation of the acylperoxo-iron(III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species. Furthermore, an unusual effect of solvent on the kinetics of oxo-iron(IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.

Mechanistic Studies on Peroxide Activation by a Water-Soluble Iron(III)–Porphyrin: Implications for OO Bond Activation in Aqueous and Nonaqueous Solvents

The reactions of a water-soluble iron(III)-porphyrin, [meso-tetrakis(sulfonatomesityl)porphyrinato]iron(III), [Fe(III)(tmps)] (1), with m-chloroperoxybenzoic acid (mCPBA), iodosylbenzene (PhIO), and H(2)O(2) at different pH values in aqueous methanol solutions at -35 degrees C have been studied by using stopped-flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo-iron(IV)-porphyrin pi-cation radical [Fe(IV)(tmps(*+))(O)] (1(++)) at pH<5.5 to the oxo-iron(IV)-porphyrin [Fe(IV)(tmps)(O)] (1(+)) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5-7.5. The observed reactivity pattern correlates with the E degrees' versus pH profile reported for 1, which reflects pH-dependent changes in the relative positions of E degrees'(Fe(IV)/Fe(III) ) and E degrees'(P(*+)/P) for metal- and porphyrin-centered oxidation, respectively. On this basis, the pH-dependent redox equilibria involving 1(++) and 1(+) are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water-insoluble iron(III)-porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe(III)(P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]-catalyzed oxidation reactions are briefly addressed.

Influence of an Extremely Negatively Charged Porphyrin on the Reversible Binding Kinetics of NO to Fe(III) and the Subsequent Reductive Nitrosylation

The polyanionic, water-soluble, and non-micro-oxo dimer-forming iron porphyrin (hexadecasodium iron 54,104,154,204-tetra-t-butyl-52,56,102,106,152,156,202,206-octakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin), (P16-)FeIII, with 16 negatively charged meso substituents on the porphyrin was synthesized and fully characterized by UV-vis and 1H NMR spectroscopy. A single pKa1 value of 9.90 +/- 0.01 was determined for the deprotonation of coordinated water in the six-coordinate (P16-)FeIII(H2O)2 and as attributed to the formation of the five-coordinate monohydroxo-ligated form, (P16-)FeIII(OH). The porphyrin complex reversibly binds NO in aqueous solution to yield the nitric oxide adduct, (P16-)FeII(NO+)(L), where L = H2O or OH-. The kinetics for the reversible binding of NO were studied as a function of pH, temperature, and pressure using the stopped-flow technique. The data for the binding of NO to the diaqua complex are consistent with the operation of a dissociative mechanism on the basis of the significantly positive values of DeltaS and DeltaV, whereas the monohydroxo complex favors an associatively activated mechanism as determined from the corresponding negative activation parameters. The rate constant, kon = 3.1 x 104 M-1 s-1 at 25 degrees C, determined for the NO binding to (P16-)FeIII(OH) at higher pH, is significantly lower than the corresponding value measured for (P16-)FeIII(H2O)2 at lower pH, namely, kon = 11.3 x 105 M-1 s-1 at 25 degrees C. This decrease in the reactivity is analogous to that reported for other diaqua- and monohydroxo-ligated ferric porphyrin complexes, and is accounted for in terms of a mechanistic changeover observed for (P16-)FeIII(H2O)2 and (P16-)FeIII(OH). The formed nitrosyl complex, (P16-)FeII(NO+)(H2O), undergoes subsequent reductive nitrosylation to produce (P16-)FeII(NO), which is catalyzed by nitrite produced during the reaction. Concentration-, pH-, temperature-, and pressure-dependent kinetic data are reported for this reaction. Data for the reversible binding of NO and the subsequent reductive nitrosylation reaction are discussed in reference to that available for other iron(III) porphyrins in terms of the influence of the porphyrin periphery.

Synthesis of Nitrosylruthenium Complexes Containing 2,2‘:6‘,2‘ ‘-Terpyridine by Reactions of Alkoxo Complexes with Acids

Nitrosylruthenium complexes containing 2,2':6',2"-terpyridine (terpy) have been synthesized and characterized. The three alkoxo complexes trans-(NO, OCH3), cis-(Cl, OCH3)-[RuCl(OCH3)(NO)(terpy)]PF6 ([2]PF6), trans-(NO, OC2H5), cis-(Cl, OC2H5)-[RuCl(OC2H5)(NO)(terpy)]PF6 ([3]PF6), and [RuCl(OC3H7)(NO)(terpy)]PF6 ([4]PF6) were synthesized by reactions of trans-(Cl, Cl), cis-(NO, Cl)-[RuCl2(NO)(terpy)]PF6 ([1]PF6) with NaOCH3 in CH3OH, C2H5OH, and C3H7OH, respectively. Reactions of [3]PF6 with an acid such as hydrochloric acid and trifluoromethansulforic acid afford nitrosyl complexes in which the alkoxo ligand is substituted. The geometrical isomer of [1]PF6, trans-(NO, Cl), cis-(Cl, Cl)-[RuCl2(NO)(terpy)]PF6 ([5]PF6), was obtained by the reaction of [3]PF6 in a hydrochloric acid solution. Reaction of [3]PF6 with trifluoromethansulforic acid in CH3CN gave trans-(NO, Cl), cis-(CH3CN, Cl)-[RuCl(CH3CN)(NO)(terpy)]2+ ([6]2+) under refluxing conditions. The structures of [3]PF6, [4]PF6.CH3CN, [5]CF3SO3, and [6](PF6)2 were determined by X-ray crystallograpy.

Mechanistic Studies on the Nitrite-Catalyzed Reductive Nitrosylation of Highly Charged Anionic and Cationic FeIII Porphyrin Complexes

The nitrosyl complexes formed during the binding of NO to the (Pn)FeIII(H2O)2 (n = 8+ and 8-) complexes, viz., (P8-)FeII(H2O)(NO+) and (P8+)FeII(H2O)(NO+), undergo subsequent reductive nitrosylation reactions that were found to be catalyzed by nitrite, which was also produced during the reaction. The effect of the nitrite concentration, pH, temperature, and pressure on the nitrite-catalyzed reductive nitrosylation process was studied in detail for (P8-)FeIII(H2O)2, (P8+)FeIII(H2O)2, and (P8+)FeIII(OH)(H2O), from which rate and activation parameters were obtained. On the basis of these data, we propose mechanistic pathways for the studied reactions. The available results favor the operation of an innersphere electron-transfer process between nitrite and coordinated NO(+). By way of comparison, the cationic porphyrin complex (P8+)FeIII(L)2 (L = H2O or OH-) was found to react with NO2(-) to yield the nitrite adduct (P8+)FeIII(L)(NO2)(-)). A detailed kinetic studied revealed that nitrite binds to (P8+)FeIII(H2O)2 according to a dissociative mechanism, whereas nitrite binding to (P8+)FeIII(OH)(H2O) at higher pH follows an associative mechanism, similar to that reported for the binding of NO to these complexes.